An inverter device connected to a three-phase power supply in use is provided with a fan which serves to cool the inverter device and which is designed to be fed with electrical power directly from the three-phase power supply. In this case, there is a possibility of error in a phase sequence when the three-phase power supply is connected to power-supply terminals for the fan. Upon occurrence of erroneous connection, the fan is reverse rotated with the result of reduction in a cooling efficiency. In view of this problem, a switching circuit is conventionally provided in addition to a power-supply switch. The switching circuit is capable of detecting and switching a phase sequence.
In the conventional switching circuit, however, there is a possibility of interphase short circuit of the three-phase power supply depending upon a state of the circuit when a relay used for switching the phase sequence malfunctions or causes welding or the like due to oscillation or noise.